Make tox run doctests

Use 'nosetests --with-doctests' to run any doctests found. We currently
only use doctests in a handful of places, but we may as well run them
to ensure they work.

Make the cfg doctests avoid using the global CONF since we would need
to reset its state between each doctest.

Fix the cliutils doctests to actually pass.

Use 'nosetests --exclude-dir=tests/testmods' to avoid loading the
modules from this dir while discovering doctests. The cfg unit tests
rely on these modules not having been previously loaded.

Change-Id: I19ad70767fa5c8352b994dc963b5d3a7c9d9eb95
Merge "Make tox run doctests"